Digital radiography (DR) systems are enjoying growing acceptance as clinical imaging tools. As shown in the simplified block diagram of FIG. 1, radiation from x-ray source 12 in a lens-coupled DR imaging apparatus 10 is passed through a subject 14 and impinges upon a scintillator screen 16, converting the energy from ionized radiation into light radiation having different frequencies, typically within the visible spectrum. In lens-coupled DR imaging apparatus 10, this emitted light energy is directed, through a lens system 18, to an image sensing apparatus 20 that then forms a digital image from the emitted light. Unlike conventional x-ray film apparatus, DR imaging apparatus 10 does not require a separate processing area or image processing consumables. Another advantage of DR imaging technology is speed, since images are obtained immediately after the x-ray exposure. For medical applications, an image can be provided to medical personnel while a patient is still present at an imaging facility.
While there are inherent advantages to DR imaging, however, there is a need to improve the overall performance of lens-coupled DR systems. One area of particular interest relates to the amount of light that lens system 18 channels from scintillator screen 16 to image sensing apparatus 20, commonly characterized in terms of optical coupling efficiency. As a rule, optical efficiency directly affects the image quality of the DR system. Improvements in optical coupling efficiency may result in improved diagnostic capability and can advantageously also reduce radiation dosage requirements in many cases.
To date, conventional lens-coupled DR systems, as shown in FIG. 1, have exhibited relatively low optical coupling efficiencies for a number of reasons. Referring to FIG. 2, light emission from a pixel 24 on scintillator screen 16 is divergent. Thus, a large fraction of the light from scintillator screen 16 is emitted at angles that exceed the light-gathering capability of standard lens components of lens system 18. Also, as shown in FIG. 3, the size of lens system 18 is often considerably smaller than scintillator screen 16 due to cost, space, and manufacturing constraints. Rays emitting from the outer regions of scintillator screen 16, such as pixels 25 and 25′, even at small emission angles, totally miss lens system 18. Thus, for the reasons shown in FIGS. 2 and 3, a large portion of the emitted light from scintillator screen 16 never reaches image sensing apparatus 20.
There have been a number of efforts at improving the optical coupling efficiency of lens-coupled DR imaging systems. One approach is directed to reducing the angular spread of the emission by controlling the structure of scintillator screen 16 itself. Examples of this approach include the following:                U.S. Pat. No. 5,519,227 (Karellas) discloses a laser-based micro-machining process for reducing spatial dispersion and scattering;        U.S. Pat. No. 5,418,377 (Tran et al.) discloses another method for treating phosphorus sites on the scintillator screen to reduce scattered luminescent radiation; and,        U.S. Patent Application Publication No. 2004/0042585A1 (Nagarkar et al.) discloses processing a columnar structured material to reduce crosstalk and enhance collection efficiency. Medical Physics Journal article “Scintillating fiber optic screens: a comparison of MTF, light conversion efficiency, and emission angle with Gd2O2S:Tb screens” Vol. 24, Number 2, February, 1991, pages 279-285, discloses scintillating fiber optic screens having forward-directed emission distributions.        
Another approach for improving optical coupling efficiency has been to improve the performance of collection optics themselves. As shown in FIG. 2, the emissive surface of scintillator screen 16 broadcasts light over a wide range of angles. Any type of collection optics must collect as much of the emitted light as possible and direct the light to image sensing apparatus 20, while keeping crosstalk between pixels to a minimum. As one example of an approach to improving collection optics, U.S. Pat. No. 6,178,224 (Polichar et al.) discloses the use of an emission modification layer positioned near the scintillation layer to limit the divergence of the emitted light. As embodiments of this emission modification layer, the Polichar et al. '224 disclosure mentions using various types of brightness enhancement film (BEF) and lenslet array or microsphere array structures.
While the approach described in the Polichar et al. '224 disclosure may improve total brightness from increased optical coupling, however, there are drawbacks for imaging when using the particular types of solutions proposed. In particular, with any of the disclosed embodiments of the Polichar et al. '224 disclosure, an increase in brightness comes at the price of lost contrast. This is because the predominant contribution to brightness increase for BEF components comes not from the BEF's divergence narrowing action, but from recycling of light rays that have undergone total internal reflection (TIR), which causes undesirable pixel crosstalk. This is illustrated in greater details by ray trace plots in FIGS. 5A and 5B. FIG. 5A shows how light from different points on the surface of scintillator screen 16 is directed through a BEF 26 having light-redirecting prisms 28, as disclosed in Polichar et al. '224. Light rays R from pixel P1, initially emitted over a fairly broad range of angles, are conditioned by BEF 26 and redirected toward normal, so that the divergence angle of light decreases from the original divergence angle shown as α1 to a smaller divergence angle α2. On the other hand, light rays from pixel P2 does not get redirected toward normal, in fact emerging from BEF 26 at an angle β2 larger than the original divergence angle β1. It can be observed that BEF 26, then, produces narrowing of the divergence of the emitted light from scintillator screen 16 only for certain pixel locations, depending on the relative positions of the pixels with respect to the light-redirecting prisms 28. Even for those light cones where divergence narrowing takes place, such as from pixel P1, the centroid remains forward-directing and does not get bent toward the center of lens system 18, contrary to what is discussed in Polichar et al. '224. A substantial part of the light cone still misses lens system 18, as shown in FIG. 5A. It is thus clear that the divergence narrowing action of the BEF 26 by itself is limited in effectiveness in increasing light throughput.
FIG. 5B shows how total internal reflection (TIR) in BEF, as used in Polichar et al. '224, decreases contrast. Light from pixel P3 enters BEF 26 at a number of angles. Some rays (R3 and R4) undergo refraction at the exit surface of BEF 26 and can be imaged by lens system 18 to form the image of P3 on image sensing apparatus 20 (not shown in FIG. 5B). Other rays undergo TIR at surfaces of BEF 26, and backscatter from scintillator screen 16 at positions different from P3. These recycled rays (R1 and R2) re-emerge from BEF 26 to be directed to image sensing apparatus 20 by lens system 18. However, R1 and R2 will be imaged to image sensing apparatus 20 at points other than the image of P3; this constitutes undesirable pixel crosstalk that degrades image contrast.
Emission modification layers using lenslet or microsphere array structures, as disclosed in Polichar et al. '224, suffer similar deficiencies as using BEF 26.
Thus, it can be seen that while solutions for improved optical coupling have been proposed, there is room for improvement, particularly with respect to improving optical coupling efficiency while maintaining image contrast.